Obesity is of epidemic proportions and adipose tissue (fat) is central to the manifestation of the accompanying metabolic syndrome (diabetes, cardiovascular disease, atherosclerosis, and hypertension). Fat tissue controls metabolism, insulin sensitivity, and temperature regulation, and is a buffer against trauma and a regulator of reproduction and satiety. The global prevalence of obesity is approximately 15% and in the United States more than two thirds of the population is considered overweight. Fat mass can expand from 2-3% to 60-70% of overall body weight in response to caloric excess. Fat tissue is comprised of two cell types: the cells that store lipids, adipocytes, and fat stem cells which can divide and have the potential to make new adipocytes. Fat accumulation occurs in two ways, one is to increase lipid storage in existing adipocytes and the second is to expand the fat stem cell population in turn creating new adipocytes. Despite the important health implications of obesity and the metabolic syndrome our understanding about adipocyte number and size determination, fat development and deposition (adiposity) and the link between these parameters and metabolic dysfunction are limited. Our lab recently identified and localized fat stem cells to the blood vessel wall in the adipose tissue thus serving as the microenvironment. Much remains unknown about these newly identified stem cells including controlling fat stem cell proliferation. To examine this and other questions, two important genes involved in controlling cell proliferation, p16Ink4a/p19Arf, were specifically deleted in mouse fat stem cells.I hypothesis that increasing fat stem cell number will prevent fat formation, prevent metabolic dysregulation and promote blood vessel formation. Preliminary findings revealed that deleting p16Ink4a/p19Arf increased fat stem cell proliferation and blocked adipocyte formation leading to reduced fat content; however, these mice display better insulin responses. Conversely, increasing the caloric intake promoted adipocyte formation reestablishing the adipose depot. Isolation of p16Ink4a/p19Arf fat stem cells from the adipose tissue and placement into cell culture revealed that they can differentiate into mature functioning adipocytes. Aim 1 will focus on understanding adiposity and metabolism under normal and obese settings. In aim 2 the emphasis will be placed on the molecular underpinnings of how p16Ink4a/p19Arf control fat stem cell proliferation and differentiation. At the conclusion of these studies, I will have expanded ou understanding of how adipose stem cell number controls adiposity and metabolism under normal physiology and obesity and may uncover new targets for metabolic therapy.